The invention relates to a communication line interface circuit for use in a switching office and more particularly relates to a telephone line interface circuit adapted to detect open and closed circuit conditions of the line.
In telephone systems, line circuits serve as an interface between communication lines connected to subscriber sets and the telephone switching network. Traditionally, the line circuit has been used to provide battery to the line causing current flow when the subscriber set is in the off-hook condition, which current flow may result in the operation of a relay or activation of a sensing device at the switching office. After an on-hook to off-hook transition has been detected at the switching office, power required for signaling and talking over the subscriber line and further supervision of the subscriber line is customarily transferred to other circuits in the switching office. Since, in many instances, telephone lines are physically located in the vicinity of electrical power lines, so-called longitudinal currents are frequently induced in a subscriber line and detection circuits connected to subscriber lines must be carefully balanced and adjusted to avoid the induced currents. It has been recognized that the effect of the longitudinal currents may be minimized by isolating the subscriber line from ground. In that case, a transformer-coupled power supply may be used. Even with a floating circuit, a ferrod sensor or the like commonly used in the current art, for example, in the No. 1 ESS Telephone Switching System manufactured by Western Electric Company, or even a relay could be used. However, such devices are costly.